The present disclosure relates generally to a temperature measurement device, and particularly to a cold junction compensation system for a temperature measurement device.
Temperature measurement devices, also referred to as temperature indicator or calibration devices, are available that operate by receiving a signal from a thermocouple and converting that signal into a temperature reading. The thermocouple signal is representative of the temperature at the thermocouple junction, referred to as the hot junction. Thermocouples operate according to the Seebeck effect (thermoelectric potential), where current is produced in a closed circuit of two dissimilar metals if the hot and cold junctions, the cold junction is also referred to as the reference junction, are maintained at different temperatures. The voltage between the hot and cold junctions is proportional to the temperature difference between the hot and cold junctions. Thermocouples may be made from a variety of materials, such as iron, constantan, copper, and tin, for example. The type and gauge size of the material used for the thermocouple is typically classified by a letter code, such as T, L, and K, for example, which typically have different voltage-temperature characteristics. Accordingly, temperature measurement devices designed for working with multiple types of thermocouple wires include a means for accommodating the different voltage-temperature characteristics.
When a measuring system is constructed using a thermocouple, one junction, the hot junction, is located at the position where the temperature is to be measured, but a spurious second junction, the cold junction, is inevitably formed where the dissimilar material wires are terminated at the measuring instrument. Since the cold junction, having thermocouple wires and contacts of different materials, is electrically connected in series with the hot junction and the temperature measurement device, an additional thermoelectric potential is introduced at the cold junction that is seen by the temperature measurement device. For instrument accuracy, this cold junction potential needs to be compensated for, which is referred to as cold junction compensation. Some cold junction compensation methods involve the use of large cold junction contacts, which produce a large thermal inertia to equalize the thermal response rate of the cold junction and the cold junction temperature sensor. However, contacts of such a large mass are undesirable for lightweight hand-held devices. Other cold junction compensation methods involve the use of thermally conductive and electrically insulative material such as silicon grease or brittle mica for conducting heat from the cold junction contacts to the cold junction temperature sensor. However, such heat conducting materials are undesirable for rapid assembly and mass production methods. Yet other cold junction compensation methods involve the use of a lookup table that is calibrated to provide an offset voltage for a given thermocouple wire type at a predefined temperature, such a zero degree-Celsius for example. However, such lookup table methods are impractical for lightweight hand held devices not having a cold junction temperature maintained at zero degree-Celsius. Accordingly, there is a need in the art for a temperature measurement device that overcomes these drawbacks.